We can’t say if avast has a bug “just” for the behaviour seen in your system.

For example, if some remnant of previous security tools is still causing some conflict with your current avast, then who is really causing this? Is it avast, or the remnants? In such case, disabling avast’s shields you see the same effect, but avast may not be the culprit.

Some other situations similar to that one are possible. Don’t get me wrong; it is still possible that avast has a bug. The point is that we need to find out where exactly the real culprit is located, instead of just showing some general consequences.

The removal utilities for older security tools that were already uninstalled was already suggested. A clean up of all your previous versions of avast was also suggested. If you want to report about these steps, we will try to continue with you narrowing down the source of problem.